The 3rd Generation Partnership Project (3GPP) oversees and governs 3rd Generation (3G) networks, including 3G Long Term Evolution (LTE) networks. 3G LTE provides mobile broadband to User Equipment (UE) within the 3G LTE network at higher data rates than generally available with other networks. For example, the air interface for 3G LTE, Evolved Universal Mobile Telecommunication System (UMTS) Terrestrial Radio Access Network (E-UTRAN), utilizes multi-antenna and multi-user coding techniques to achieve downlink data rates of 100s of Mbps and uplink data rates of 10s of Mbps.
In LTE E-UTRAN, user mobility is controlled by the network with assistance from the UE. Handover decisions, as well as the choices for the target cell and technology (when applicable), are made by the current serving Evolved Node Base Station (eNodeB), based on measurements made by the eNodeB, and also based on measurements reported by the UE to the eNodeB. Due to the nature of E-UTRAN, the number of packets buffered before scheduled transmissions occur following the handover may not be negligible. For that reason, packet forwarding mechanisms may be used, when applicable, between a pre-handover “source” node (e.g., eNodeB) and a post-handover “target” node (eNodeB, or if the radio access changes from E-UTRAN to 2G/3G, a Radio Network Controller, or RNC) so as to limit packet loss during handover from the source node to the target node.
LTE E-UTRAN provides several interfaces to reduce packet loss during handover from a source eNodeB to a target eNodeB. The X2 interface provides a direct forwarding path between the two eNodeBs. The user plane protocol stack on the X2 interface uses GPRS Tunneling Protocol User data (GTP-U) tunneling. The X2 interface is optional, and may not be available between a given source and target eNodeB. In this case, an indirect forwarding path exists over an S1-U interface between source eNodeB and source serving gateway (SGW); through network connectivity between the source SGW and target SGW; and over S1-U from the target SGW to the target eNodeB. Like X2, the user plane on the S1-U interface uses GTP-U. The network connectivity between the source and target SGW may be via a network node such as a packet data network gateway (PGW), with connections to the SGWs over an S5 (non-roaming) or S8 (roaming) interface.
Regardless of whether the data is forwarded via a direct or indirect forwarding path, the target eNodeB receives forwarded, pre-handover data packets from the source eNodeB, as well as post-handover data packets from the core network (e.g., PGW). Delays in the forwarding path may cause the target eNodeB to receive, and hence deliver to the UE, data packets that are out of order.
The 3GPP Specifications (e.g., TS 36.300 and TS 23.401) define a GTP-U “end marker” (GTP-U message type 254—referred to herein as an “end marker packet” or simply an “end marker”) that delineates the end of forwarded packets, and hence prevents out-of-order delivery, for example in handover from a source eNodeB to a target eNodeB, when both are anchored by the same SGW. The GTP-U end marker mechanism may also be used across the S5/S8 interfaces, when they are implemented with the GTP-U protocol. However, in some networks, the S5/S8 interfaces are based on the Proxy Mobile IPv6 (PMIP) for the control plane, and Generic Routing Encapsulation (GRE) for the user plane. No end marker mechanism is defined for the PMIP-GRE protocol. Accordingly, in an intra-EUTRAN handover with SGW relocation, at least where the S5/S8 interfaces use PMIP-GRE, the target eNodeB may deliver data packets to a UE out of order.
One possible solution to this problem would be to insert sequence numbers in the header of each data packet. While such sequence numbers would enable the target eNodeB to determine the correct order for transmitting the data packets to a UE, using such sequence numbers would undesirably increase the overhead and signal processing associated with the transmission of each data packet.